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Metal halide and HPS lamps

HID lamps still set the intensity bar in many rooms. MH is whiter and carries UV. HPS is amber, efficient, and strong in flower. Digital ballasts waste less power than magnetic ones.

light flowering vegetative canopy temperature
Metal halide and HPS lamps — Marijuana Grower's Handbook, page 237
Full book

Metal halide

Look

White sports-field light. Less total output than HPS, so many growers no longer reserve MH for veg only

PAR share

About 15% of MH energy is PAR versus about 13% for HPS

Red gap

Standard MH is low in red, which plants want for photosynthesis and flower, but MH still out-buds fluorescents and punches a bit deeper

Discretion

White light is less obvious outdoors or in a window than amber HPS, and it adds useful UVB

Coverage

250 W about 2×2 ft; 400 W about 3×3 ft; 1,000 W about 4×4 ft

Broken jacket

Unplug immediately. Exposed MH emits harmful UVB and deadly UVC. Do not remove the outer glass on purpose

High-pressure sodium

Look

Example strains
Orange-amber streetlight. Heavy yellow orange and red

Flower use

AromaExtra orange/red is why growers favor HPS for bloom

Sizes

Example strains
Common garden lamps are 150 250 400 600 750 and 1 000 W. 600 W and 750 W are about 7% more efficient than the others

Output

AromaAbout 15% more light than MH. Same remote-ballast hanging style

Coverage

Aroma250 W about 2×2 ft

Spacing

AromaAbout 1 m (3 ft) apart gives roughly 60 W/sq ft (660 W/m²)

Veg, UVB finish, and stretch myth

All stages

HPS can run veg and flower with no extra lamp if you accept the spectrum

Blue-enhanced HPS

Some bulbs add blue to keep stems shorter

Last two weeks

Swapping to MH for the finish can raise THC if UV actually reaches the canopy

Air-cooled glass

Protective glass eats UV, so UVB from MH only counts if the glass is off and the lamp is cooled another way

Stretch myth

HPS red does not cause stretch. Stretch is far-red. HPS does not emit those wavelengths. Incandescent light looks similar but is rich in far-red

Ballasts and lamp age

Job

AromaThe ballast steps house current up to lamp voltage. Hang only the reflector

Magnetic

Example strains
Tied to one lamp type wattage and grid voltage. About 20% extra draw heavy (35-54 lb for 1 000 W) noisy hot

Digital

Example strains
Often runs MH or HPS of the same watt 110 or 220 V about 10% overhead 8-12 lb silent softer start less flicker longer lamp life 15-20% more light

1,000 W math

AromaMagnetic plus lamp can exceed 1,200 W at the wall

Retire bulbs

AromaReplace after about 8-9 months of flower use, or about 5 months if the lamp runs continuously. Compare to a new bulb or use a meter